The electronics industry and integrated circuits seem to be inversely related. The industry grows as circuits shrink and growth continues as long as more and more circuits can be integrated into a single chip. The motive for shrinking the components of integrated circuits is minimizing the cost and time needed to perform each circuit function. Most functions are carried out by transistors which act essentially as switches. In a transistor, the speed and precision with which switching can be controlled, as well as the power needed to produce the switching, is directly related to the time and cost per function attained by the device. Because of its size, a transistor switch that operates on the principles of quantum mechanics would be faster and would consume less power than a conventional transistor. Furthermore, because of the effects peculiar to quantum phenomena, it could also afford a greater degree of control.
Because the way in which quantum semiconductor devices function is qualitatively different, quantum-effect devices promise more precise and efficient control of switching in a size regime ordinary transistors and other semiconductors devices could never approach. This difference is manifested for example in the current-voltage characteristics. In particular, some quantum semiconductor devices exhibit negative differential resistance: that is, there is a voltage range in which the current decreases as the applied voltage increases. On a graph of current versus voltage, this property translates into a current peak and a current valley. The presence of negative differential resistance is often the only indication a physicist has that quantum-effects are operative in an experimental device.
The elusive phenomenon at the heart of quantum-effects is the wave nature of electrons. Quantum theory predicts that an electron will exhibit wave-like behavior whenever the region within which it is confined, or the barriers erected to contain it, has dimensions approaching the electron's wavelength.
Hence, at least one dimension of a quantum device is comparable to the wavelength of an electron. By way of example, in gallium arsenide at room temperature that wavelength measures about 200 Angstrom units or 20 billionths of a meter.
The barriers that can contain electrons are barriers of energy rather than physical barriers. All electrons possess a finite amount of energy and are said to occupy energy levels; the levels available are characteristic of a given material. A group of closely spaced levels is called a band. In most solids the energy levels in each band are so closely spaced that they are essentially continuous and thus an electron can change levels with only an infinitesimal boost of energy.
The relative positions of energy bands determine whether current can be conducted across an interface of two different materials. For an electron to pass from one material to another without change of energy, the bands of the two materials must overlap. Specifically, in the first material the average level occupied by electrons, called the Fermi level, must coincide with an energy band of the second material. If the energy band of the second material occurs at a much higher energy level than the Fermi level of the first, the second material acts as a barrier to electron movement.
For example, under ordinary circumstances, aluminum gallium arsenide (AlGaAs) presents a barrier to the electrons in n-doped gallium arsenide. An electron cannot pass from the doped gallium arsenide (GaAs) to aluminum gallium arsenide because the conduction band of aluminum gallium arsenide is at a much higher energy level than the Fermi level of the gallium arsenide. Yet, if the physical dimensions of the barrier are altered in such a way that the wave nature of electrons comes into play, an electron can "tunnel" through the aluminum gallium arsenide that was once an obstacle. Hence, when a layer of aluminum gallium arsenide thinner than 200 Angstroms is sandwiched between two pieces of doped gallium arsenide, the electrons can tunnel through it to the gallium arsenide on the other side. This tunneling is one kind of quantum-effect. When barriers confine electrons within a space comparable to an electron wavelength, the electrons are subject to other quantum-effects.
Resonance is one of the other such quantum-effects and occurs only when some degree of size quantization has been achieved. Electron waves, at a given energy, that enter for example, a quantum well, which is a one-dimensional restriction, are reflected off the far wall of the well; the waves essentially bounce back and forth within the quantum chamber. In doing so they increase the tunneling current substantially, i.e., they resonate. Both size quantization and resonance result from the constructive interference of the forward and backward waves. It is difficult to separate the current enhancement that can be attributed to resonance from the enhancement that results from other quantum-effects such as increased density of states at a given energy level. However, as it happens, that distinction is not crucial for device operation. What does matter is that in a quantum-effect device two slightly different voltages can evoke profoundly different responses. For example, the current-voltage characteristics of a quantum well device reflect the quantization of energy states in the gallium arsenide well. Such devices show a range of voltages in which the current conducted by the device decreases as the voltage applied to one of the n-doped gallium arsenide contacts increases. This happens because at one voltage (the resonant voltage) the average energy of electrons in the n-doped substance shifts to a level that coincides with one of the quantum states in the well, but beyond that voltage the energy band of the doped gallium arsenide occurs between quantum states. Hence, at the resonant voltage an electron can tunnel through the aluminum gallium arsenide energy barrier into the well, whereas at the valley voltage, there are no states for the electron to tunnel into and consequently, the current dips dramatically.
Semiconductor tunnel structures with negative differential resistance (NDR) have been extensively studied for years. The reason for this interest are applications such as microwave and fast digital devices. The tunnel diode (the ESAKI DIODE), in which carriers tunnel across the band gap of a forward biased pn-junction, was invented in 1958. In 1974 the double barrier resonant tunneling diode was first demonstrated. In this structure NDR arises from electrons tunneling through a quantum well state. Recently, the single barrier tunneling diode was introduced as another NDR device. Electrons tunnel through this structure at energies close to the valence band of the barrier layer yielding a decrease in transmission probability as the voltage increases.
For many applications, NDR devices must have a large peak current density and a low valley current density. Hence, the peak-to-valley current ratio is used as a figure of merit. The ESAKI diode has produced peak-to-valley current ratios larger than 50. Aluminum gallium arsenide/gallium arsenide double barrier resonant tunneling diodes have been observed to display a peak-to-valley current ratio of 3.9 at room temperature and 21 at 77 degrees Kelvin, the temperature of liquid nitrogen. Similarly, indium gallium arsenide/aluminum arsenide structures have yielded peak-to-valley current ratios of 14 at room temperature and 35 at the temperature of liquid nitrogen. The single barrier tunneling diode structure of indium arsenide/aluminum gallium antimonide has thus far produced peak-to-valley current ratios of 1.6 at room temperature and 3.4 at the temperature of liquid nitrogen. The higher is the peak-to-valley current ratio of a quantum-effect semiconductor device, the more useful its applications may be such as in the form of diodes and transistors. By way of example, an extremely high peak-to-valley current ratio for use in a diode, would find highly advantageous application to microwave oscillators, mixers and detectors as well as switching applications and high speed digital and analog circuits. Devices which employ extremely high peak-to-valley current ratios in transistor form find advantageous application in microwave amplifiers and also in digital and analog high speed circuits.